Question

A train, standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air. (i) What is the frequency of the whistle for a platform observer when the train (a) approaches the platform with a speed of 10 m s–1, (b) recedes from the platform with a speed of 10 m s–1? (ii) What is the speed of sound in each case? The speed of sound in still air can be taken as 340 m s–1.

Answer 1

(1) (a) here,
frequency of whistle ( f) = 400 Hz
Speed of train ( Vt) = 10 m/s
Speed of sound ( V) = 340 m/s

Use Doppler's effect of sound

Apparent frequency ( f')= f × { V/(V-Vt)}
= 400 × { 340/(340 -10)}
= 400 × 340/330
= 412.12 Hz

(b) when the train recedes
Apparent frequency ( f") = f× { V/(V+ Vt) }
= 400 × { 340/(340+10)}
= 400 × 340/350
= 388.57 Hz


(II) speed of the sound depends the property of the medium . in either case the medium property doesn't change so, the speed of the sound remains constant .

Answer 2

Answer:

Frequency=400Hz

Speed of train=10m/s

Speed of sound=340m/s

Explanation:

a) 412.12hz

b) 388.57hz

c) Speed of sound remains constant in each case